At present, most of the commercial relays, when provided with an operation current, can change their output states, e.g. from "on" to "off" or from "off" to "on", and will return to their original states when the current provided disappears. In order to maintain the changed states, the current must be provided constantly. An improved relay is on the market to overcome this drawback which needs only a single pulse to keep the changed state. The most relative prior art is the latch relay manufactured by COLE HERSEE CO., U.S.A., but it is complicated, large, and unstable in operation since it requires three springs.